Is it OK to put standard "twin & earth" cable directly into the cavity of a dwarf wall?

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Last night, I ran the cables for my ring main into the cavity of the dwarf wall in my conservatory (before the plastic went on).. Should I do anything to protect the cable, as it seems a little 'exposed'.
I was recently reading about problems with "damp" when the cavity is breached.. When looking at the 'bat ties' which run between the cavity, I noticed that they have little kinks the shaft, to prevent water from running down them.
Because the cable is obviously not 100% straight, it touches both of the walls at various points.. Could this be a source of damp?
I will be putting some insulation into the cavity, but should be doing anything else to protect the cables?
Any info on this would be apprecaited
Jon
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You may well find it bridges the cavity. It is usually better to chase the inside wall and plaster over. If you must run in there, you should use cable clips to fix to the inner (dry) leaf. The bricks are over 50mm wide, so you can run the cable at any height. I'd suggest near the top where you can swing the hammer for the clips. You must derate the cable for the expected insulation, which may require a ring main to be run in 4mm (or even 6mm in some cases) after the calculations.
Christian.
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AIUI, 2.5mm cable run in contact a conductive surface with the rest of the cable completely surrounded by insulation is rated at 21A which is sufficient for a 32A MCB protected ring main. This is all in the OSG and was discussed here fairly recently (don't have the OSG to hand so can't look up the table number for you). The only other thing you *might* have to take into account is the total length of the circuit, but that is very rarely a problem in a "normal" house!
Hwyl!
Martin.
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Martin Angove: http://www.tridwr.demon.co.uk /

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I don't have my OSG on me either. However, I seem to recall a 50% reduction for being buried in insulation, which is what you would have to use if using the OP's original idea of letting it float within the cavity, not clipped. This would mean his original plan would be seriously non-compliant, quite apart from the cavity bridging issue.
Christian.
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But it is reduced from 27A "clipped direct". This is the *new* table 4D5A in the regulations or 6F in the OSG. Note that 21A is given with "method 15" - where the cable is partly surrounded by insulation.
Sorry, brief message. My experiment with NS6 on this G3/300MHz/64M/OS9 Mac using Google is rather sss-lll-ooo-www :-/
Hwyl!
M.
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(As a reader only): http://www.livtech.co.uk /
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I was originally going to run the circuit as a 'radial spur'. As I have 4 double sockets, I was going to install a 13A fuse spur off the existing ring and then just run a radial circuit from the spur to the sockets.
But, considering that I want to put a 3kW heater in, the conservatory, then thats 12.5A already..
So I decided on a ring.. I do have a fairly large house, and extending it through the conservatory has added an extra 20m to this (10 meters each way), so I hope that this won't be a problem!
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It will, unfortunately. A 2.5mm cable buried in insulation is not suitable for a 32A ring main. If you can't redo in thicker cable (i.e. the windows have gone in), I'd run the new section back to the consumer unit and use a 20A MCB. That will be enough for your 3kW heater and a few other bits. Alternatively, run it as 2 13A spurs, with the fan heater off an FCU directly and the sockets off another fused 13A spur. You can easily do this as you ran as a ring, so you've already got two supplies to the sockets, provided the FCU for the heater can be on the first or last socket of the run.
Christian.
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I have looked it up in the regs and am not sure what you are referring to here.
I have the "IEE Wireing Regulations" for 1991 to hand (which is out of date, but I wouldn't have thought that things would change to much regarding current capacity of a conductor).
Table 4D24 covers "Multicore P.V.C Insulated Cable, Non-Armored (Copper)" and WORSE case for a 2.5mm cable is 18.5 amps (for a heat insulated installation)
As I have a ring, this will give me capability of 37amps and protecting this by a 32MCB should be fine.
Now condidering that this cable is in a cool, cavity on an external wall , I would imagine that it would be able to carry MORE than 18.5amps, but even if 18.5A was the limit, its still within spec.
In "Free Air", the same table shows that the cable will carry 30amps, so I would im
I am not talking about packing insulation around the cable, but instead, placing it fairly loosly in the cavity. As the cable is right at the bottom of the cavity (below the DPC) there would still be a air space around the cable.
But, back to your statement about a 2.5mm cable not being upto the job. From the info that I have, even if packed tightly in an insulated space, it would still be fine.
Please can you confirm where you got your info, as I am slightly worried now and want to be sure that I am right before continuing.
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"Jon Weaver" wrote in message

Hopelessly out of date in many respects, I fear. Christian has posted details relevant to ring circuts. It would be worth you buying the up-to-date OSG (blue cover).

Serious misunderstanding there: a ring cable has to be rated at 20A (formerly two-thirds of the fuse/MCB rating), and _not_ half of the fuse/MCB rating. This is allow for the fact that the load won't be uniformly distributed around the ring. Another amendment since 1991 requires that you specifically consider the likely distribution of load on every ring to ensure that the actual cable rating won't be exceed for prolonged periods. A problem could arise if (say) you had all your kitchen appliances towards one end of a long ring, leading to the need to consider a different circuit layout, or ring sections in 4mm^2 cable.

That's not a terribly sensible place to run a cable. It's an area usually full of spiky mortar snots and other brickies' debris. I'd be worried about the risk of slight movement leading to cable damage.
--
Andy



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Christian McArdle wrote in message

Agreed -- in fact it's only just OK for the 13A fused spur.

But that will gain him absolutely nothing. You're replacing a ring which requires a cable rated at 20A with a radial circuit also requiring a cable rated at 20A, whilst the cable (as instlled) only has an effective rating of 13.5A.

That's what I was going to suggest.

The heater socket or FCU needs to be on a run of its own. In fact with a single FCU or single 13A socket, it could be an unfused spur.
--
Andy



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Sorry, I wasn't clear. It would become a 20A ring. A bit non-standard, I know, but safer than running 32A.

Another misunderstanding. I'll try to explain better with some ASCII art.
OP's original plan:
+----SKT1-----SKT2------SKT3----SKT4-----+ | | ---+-// original ring cut here //-----------+-----
Proposed plan:
+----SKT1-----SKT2------SKT3 FCU------+ | | --FCU---------------------------------------+----- original ring left intact
I suppose the FCU on the right could just be an unfused spur socket for the heater. The left hand FCU could be on the ring as shown, or installed adjacent to SKT1. In either case, it does not require laying more cable.
However, I'm now getting confused as to the rating of 2.5mm cable layed within insulation. Not having regs with me doesn't help. What is the definitive current capability of it?
Christian.
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"Christian McArdle" wrote in message

Oh OK, but received wisdom is to avoid non-standard arrangement like this, especially in domestic installations.

I think we're saying the same thing really, but got there by different paths.

There's no installation Method number for "totally surrounded by thermal insulation", because it's something that you're supposed to avoid. So (IMO) you take the clipped direct rating (27A) and divide by 2, hence 13.5A.
--
Andy



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I had precisely the same issue as the OP, viz. dwarf cavity walls and wanting not to use plaster on the inside but to have brick.
I wanted to run a power ring circuit and also CAT5, CT100 and speaker cables.
Clipping the power cables within the cavity to a leaf using convention clips was not practical, so I devised a way to do this differently. I also had in mind the rules about cables needing to be in a band of 150mm down from the top of a wall and at least 50 mm below a surface.
So, I began by fitting insulating batts up to a distance of about 250mm below the top level of the wall. I ran all of the non power cables simply by laying them on the batts at this level and feeding through. I then added strips of batt to take the insulation up to a level 100mm below the top level of the wall.
I made special brackets from fairly stout galvanised strip about 8mm wide and fashioned towards one end a U-shape designed to accept the power cable. The other was bent to an L-shape, but actually slightly acute of the 90 degree angle. The distance from the U part to this bend was about 60mm, as was the remaining straight piece the other side of the L. I covered the U part in two layers of heat shrink sleeve and drilled a hole 25mm from the other end.
At each clipping position I had had the builder put in wooden blocks between the bricks to a depth down from the top of 15mm, and coming forward from the cavity side of the inner leaf by 50mm. Mortar was placed in front of these blocks so that the inside face of the inner leaf is normal. The same thing could have been done after building by carefully drilling down from the top and removing mortar.
Each clip was installed by putting it into place holding the cable, and temporarily holding with a clamp. Then the fixing hole position was marked from above. The assembly was removed and the hole drilled from above to receive a plug. The bracket was then refitted with the cable, clamping again while the screw was inserted.
The effect is that the cable is held in place against the inner leaf of the wall about 70mm down. I forget how frequently the clips were placed, but within the horizontal clipping distance.
The space in the top of 100mm was left without any insulating batt.
So in effect, the cable is clipped to a masonry surface and is at least 50mm away from any potential mechanical damage.
.andy
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Hi Andy,
Glad you said that... Following advise from earlier posters, I was tempted to bring the cables to the top of the leaf.. This would have bought them to around 20-30mm from the top of the wall, and once the 'window sills' had gone in, less than 50mm below a surface.
I spoke to our office 'electrician' and doesn't see a problem leaveing the cable in the cavity as it is.. Sure, its quite sharp down there, but there is going to be very little (if any) movement once its all finished and therefore little risk of damage in the future.
Now that the base and wall is finished, I don't really have much choice.. As I said above, I could quite easily raise them to the top of the walls, but then I would break the regulations (150mm and 50mm) that you mention..
I simply can't win!
Many thanks for the advice.
Jon
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Many thanks for this info.. Am I right in thinking that this entire debate has come about, because I mentioned "insulation"?
At the moment, the 2x2.5mm cables are simply lying in the cavity, which does currently does not have any insulation in it... Based on all of the info that I have read, I think that in this configuration (i.e cable in free air), its more than up to the jobs. And i would hope it is, as thats what the rest of my ring is wired with.
With regards to attaching the cable to the leaf.. Its no so much that I "havn't" gone to the trouble, but more that I didn't realise that I should. I too was concerned about the damage which could be done to the cable in the cavity (due to sharp edges on the morter) and one of the reasons why I originally asked this question. However, I was realiable informed by sevearal of my (apparantly well qualified) collagues that it was OK as once the cable was in, it wasn't going to move and it was VERY unlikely that any damage could or would occur at a later date.
However, this weekend I will look at the possibility of bringing the cable to the top of the leaf and fixing it in some way.. It may be an off the wall suggestion (no pun inteded), but what about fixing the cables (using cable ties) to the metal 'bat ties' which link the 2 walls together?
With regards to the electical side of this, I think that I have 3 options.
Bearing in mind that I have 4 double sockets and the items that I intend to use in the conservatory are:
2 x Lamps with 60W bulbs 1 x TV 1 x Oil Filled Radiator (2.5 or 3kW)
My choices are:
1. Leave it as it is (a 2.5mm ring) but not add any insulation to the cavity.
2. Disregard one of the cables and connect the other as a radial circuit to a via a 13amp Fused Spur - This was my original plan, but would restrict me to a total of 13amps in the conservatory.
3. Use the 2nd cable to create 2 radial circults protected by 2 x 13amp Fused spurs.. One for sockets 1 and 3, the other for 2 and 4. - This means that in theory, I have access to a total 26amps (which is completely over the top), as long as I am careful which devices to plug in which sockets.
Your thoughts on this would be appreciated
Jon
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I am SERIOUSLY confused now and to be honest, slightly worried.
As far as I know, a standard domestic ring is done with 2.5mm Twin and Earth.. If this is the case, when whats the difference between that and what I am doing now. I am simply breaking the ring at the back of an existing socket and extending it through the conservatory.
Has all of this started because I mentioned 'insulation'.. If it is, thats simple.. I won't use insulation, or I will bring the cable ABOVE the insulation.
The ring mains in my house are wired in 2.5mm and protected by a 32A MCB! Does this mean that they not up to the job and are consiquently dangerous?
If what I have done in my conservatory really isn't up to the job, then I will simply resort to plan B which was to go for a 'radial circuit' protected by a 13A spur.
With regards to the 'heater' outlet, I am not 'hard-wiring' in.. I am buying a DeLonghi oil filled radiator (2.5kW or 3kW) which AFAIK comes fitted with a standard 13amp plug, which can be plugged into any socket.
Can someone please explain, as I am a really confused now.
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Jon Weaver wrote in message

YES. If there's no thermal insultion, or if there is and the cable touches the wall on one side there's no problem at all.

OK.
No, they sound perfectly normal.

The point there was that you were proposing to put in a fused spur, knowing that you would connect a 3kW heater, so that the use of any other appliance would overload the circuit. Hence our suggestion to split it into two spurs with the heater socket on its own. But if you revert to Plan A (minus thermal insulation) then this is no longer relevant.
--
Andy



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Yes, I think other posters have got a bit lost in the "buried in insulation" thing. As long as the cable isn't *surrounded* by insulation then I think you are fine.

I don't think you need to worry! :-)
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Chris Green ( snipped-for-privacy@x-1.net)

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Hi Christian,
What you have suggested is not possible in my case.. Running cables on the inside is no good, as I am leaving the brickwork exposed and won't be plastering it over.
I could bring the cables to the top of the internal wall, but I really don't think that I will be able to clip it in place.. There just isn't enough room in the cavity to swing a hammer, even from above.. Chances are that I would end up chipping the tops of the bricks and making a right mess.
At the moment, I have pushed the cable down into the cavity as far as they would go. Effectivly they are lying below the DPC level.
I really hope that this is OK as I have already shreaded my arms and don't want to do anything more.
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Due to the insulation, the work you've already done won't be compliant. The insulation will cause the wiring to overheat. If you want to keep it, then run the circuit back to its own MCB, rated at 20A, rather than 32A. This should give better protection, in line with the 2.5mm's actual capability buried in insulation. With bare brick in a conservatory, you might not care about the bridged cavity anyway. There's no plaster to spoil.
One other technique you could use is to take thin battens and nail clips to them every 40cm. Stick these to the top of the wall inside the cavity with Gripfill, let it bond, and then push the cable under the clips, or use plastic cable ties. You'll probably still have to derate the cable, as I'm not sure if wood is a conductive surface (no OSG again!), but at least it will be tidier, simple to do and won't bridge the cavity.
The final suggestion is to use metal conduit on the inside wall. This will make it look a bit like a school or light industrial premises. However, depending on the style of the house and conservatory, this might actually look OK.
Christian.
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